The present invention relates to wireless digital networks, and in particular, to the problem of locating a wireless device.
Determining the location of a wireless device is an old problem. Typical solutions rely on a number (greater than two) of locating devices (LD) at known locations. Each LD makes a measurement of some characteristic of a signal from the device to be located (DTL), where the measurement correlates to a distance estimate between the DT and the LD. Using distance estimates and locations from three LDs, a location for the DTL may be calculated. Using distance estimates and locations from more than three LDs, an overdetermined location for the DTL may be calculated.
One measurement which may be made by the LDs is the time of arrival of a signal transmitted by the DTL. As an example, LDs may time stamp the arrival of a particular part of a known frame transmitted by the DTL. To derive an accurate location, these time stamps must be synchronized, which may be accomplished using for example IEEE-1588 Time Synchronization to synchronize clocks of LDs on a common wired network. The resulting time difference of arrival (TDoA) measurements are used to calculate a location for the DTL, as taught for example in U.S. Pat. No. 7,630,728 to Cutler, et al.
Another approach is to use the signal strength of a DTL transmission as measured by each LD. Assuming the receive path from antenna through to signal strength measurement in the LD is properly characterized, DTL signal strength at the LD antenna may be estimated. Assuming the DTL is an isotropic radiator operating under known propagation conditions, the free space path loss (FSPL) which is a function of distance and frequency can be used to estimate distance differences among three or more LDs, allowing for a calculation of DTL location. The free space path loss model can also be replaced by other models that accurately represent the RF environment such as an office or a healthcare facility.
Similarly, if the effective radiated power (EIRP) of the DTL is known, the difference between this EIRP at the DTL and the signal strength at the LD can be turned into a distance estimate using the formula for the selected path loss model. Three or more of such distance estimates from LDs may be used to calculate DTL location. The EIRP of a DTL may be estimated, by example, from looking up its MAC OUI to retrieve the manufacturer and device type of the DTL if possible, and to perform a device lookup using this information to find the device EIRP. Device EIRP may also be estimated by device class or characteristics; as an example, Bluetooth devices operating according to the standard operate at one of a fixed number of power levels.
What is needed is a way to improve distance estimation of a DTL which does not directly rely on measuring DTL signal strength or signal-to-noise ratio at the LD.